The Principles of Stratigraphical Geology

by J. E. Marr

Comparative View of the Names and Succession of the Strata.

Table drawn up in 1799.			Table accompanying the map, drawn up in 1812.		Improved table drawn up in 1815 and 1816 after the first copies of the map had been issued.
			London Clay		1	London Clay
			Clay or Brick-earth		2	Sand
					3	Crag
			Sand or light loam		4	Sand
1	Chalk		Chalk		5	Chalk Upper Lower
2	Sand		Green Sand		6	Green Sand
			Blue Marl		7	Brick Earth
			Purbeck Stone, Kentish Rag and Limestone of the vales of Pickering and Aylesbury, Iron Sand and Carstone		8	Sand
					9	Portland Rock
					10	Sand
					11	Oaktree Clay
					12	Coral Rag and Pisolite
					13	Sand
3	Clay		Dark Blue Shale		14	Clunch Clay and Shale
					15	Kelloway's Stone
			Cornbrash		16	Cornbrash
4	Sand and Stone				17	Sand and Sandstone
5	Clay
6	Forest Marble		Forest Marble Rock		18	Forest Marble
					19	Clay over Upper Oolite
7	Freestone		Great Oolite Rock		20	Upper Oolite
8	Blue Clay
9	Yellow Clay
10	Fuller's Earth				21	Fuller's Earth and Rock
11	Bastard ditto and Sundries
12	Freestone		Under Oolite		22	Under Oolite
13	Sand				23	Sand
					24	Marlstone
14	Marl Blue		Blue Marl		25	Blue Marl
15	Blue Lias		Blue Lias		26	Blue Lias
16	White Lias		White Lias		27	White Lias
17	Marlstone, Indigo and Black Marls
18	Red Ground		Red Marl and Gypsum		28	Red Marl
19	Millstone		Magnesian Limestone		29	Redland Limestone
			Soft Sandstone
20	Pennant Street
21	Grays		Coal Districts		30	Coal Measures
22	Cliff
23	Coal
			Derbyshire Limestone		31	Mountain Limestone
			Red and Dunstone		32	Red Rhab and Dunstone
			Killas or Slate		33	Killas
			Granite, Sienite and Gneiss		34	Granite, Sienite and Gneiss

The above table contains a very complete classification of the British Mesozoic rocks, one of the Tertiary strata which is less complete, and a preliminary division of the Palæozoic rocks into Permian (Redland Limestone), Carboniferous (Coal Measures and Mountain Limestone), Devonian (Red Rhab and Dunstone) and Lower Palæozoic (Killas).

Since Smith's time the main work which has been done in classification is a fuller elucidation of the sequence of the Tertiary and Palæozoic Rocks, and this we may now consider.

The Mesozoic rocks are developed in Britain under circumstances which render the application of the test of superposition comparatively simple, for the various subdivisions crop out on the surface over long distances, and the stratification is not greatly disturbed. With the Tertiary and Palæozoic Rocks it is otherwise, for some members of the former are found in isolated patches, whilst the latter have usually been much disturbed after their formation.

Commencing with the Tertiary deposits we may note that "the first deposits of this class, of which the characters were accurately determined, were those occurring in the neighbourhood of Paris, described in 1810 by MM. Cuvier and Brongniart.... Strata were soon afterwards brought to light in the vicinity of London, and in Hampshire, which although dissimilar in mineral composition were justly inferred by Mr T. Webster to be of the same age as those of Paris, because the greater number of fossil shells were specifically identical[7]." It is to Lyell that we owe the establishment of a satisfactory classification of the Tertiary deposits which is the basis of later classifications. Recognising the difficulty of applying the ordinary test of superposition to deposits so scattered as are those of Tertiary age in north-west Europe, he in 1830, assisted by G. P. Deshayes, proposed a classification based on the percentage of recent mollusca in the various deposits. It may be noted, that although this method was sufficient for the purpose, it has been practically superseded, as the result of increase of our knowledge of the Tertiary faunas, by the more general method of identifying the various divisions by their actual fossils without reference to the number of living forms contained amongst them. The further study of the British Tertiary rocks was largely carried on by Joseph Prestwich, formerly Professor of Geology in the University of Oxford.

[7] Lyell, Students' Elements of Geology. 2nd Edition, p. 118.

Amongst the Palæozoic rocks, it has been seen that the Permian, Carboniferous and some of the Devonian beds were recognised as distinct by Smith, though a large number of deposits now known to belong to the last named were thrown in with other rocks as 'killas.' The Devonian system was established and the name given to it in 1838 by Sedgwick and Murchison, largely owing to the palæontological researches of Lonsdale. An attempt was subsequently made to abolish the system, but the detailed palæontological studies of R. Etheridge finally placed it upon a secure basis. The establishment of the Devonian system cleared the way for the right understanding of the Lower Palæozoic rocks, which Sedgwick and Murchison had commenced to study before the actual establishment of the Devonian system, and to these workers belongs the credit of practically completing what was begun by William Smith, namely, the establishment of the Geological Sequence of the British strata. The controversy which unfortunately marked the early years of the study of the British Lower Palæozoic Rocks is well-nigh forgotten, and in the future the names of Sedgwick and Murchison will be handed down together, in the manner which is most fitting.

Our account of the growth of British Stratigraphical Geology is not yet complete. In 1854, Sir William Logan applied the term Laurentian to a group of rocks discovered in Canada, which occurred beneath the Lower Palæozoic Rocks. Murchison shortly afterwards claimed certain rocks in N.W. Scotland as being of generally similar age, and since then a number of geologists, most of whom are still living, have proved the occurrence of several large subdivisions of rocks in Britain, each of which is of pre-Palæozoic age.